Microlithography projection exposure apparatuses are used to produce microstructured components using a photolithography method. In this case, a structure-bearing mask, the so-called reticle, is illuminated using an illumination system and imaged onto a photosensitive layer using a projection optical unit. In this case, the minimum feature size, that is to say the resolution which can be imaged with the aid of such a projection optical unit, is determined by various factors. Firstly, the resolution is related to the wavelength of the imaging light used, in which case the smaller the wavelength of the imaging light used, the smaller the structures that can be imaged. Furthermore, the larger the numerical aperture of the imaging light at the location of the photosensitive layer, the greater the resolution.
In order to ensure this quality of the imaging, however, it is desirable for the image errors of the projection objective to be sufficiently small. This means, for example, that the wavefront aberrations of the projection objective are of the order of magnitude of a few milli-lambda (mλ), where λ is the wavelength of the imaging light used. During the operation of the microlithography projection exposure apparatus, however, various effects occur which can adversely influence the wavefront aberrations of the projection objective. Thus, e.g., various lens materials such as e.g. quartz or flint glasses, exhibit damage in the form of material compaction on account of the permanent irradiation. Stresses are thereby induced in the lens, which can lead to stress birefringence. Furthermore, all optical components, such as mirrors and lenses, have a certain residual absorption, which has the consequence that the optical elements are heated during the operation of the projection objective. This can cause a deformation of the optical elements and also a change in the refractive index due to the resulting material expansion. All these effects lead to an alteration of the wavefront aberrations of the projection objective, such that the imaging quality is adversely influenced. For this reason, a projection objective generally includes a multiplicity of correction mechanisms, such as, e.g., displaceable or tiltable optical elements, with which such system alterations can be compensated. In order to drive these correction mechanisms, however, it is desirable to measure the wavefront aberration of the projection objective very accurately. For this purpose, generally the exposure process is interrupted and, e.g., an interferometric measurement of the wavefront aberrations is performed. However, the interruption for measurement purposes can have the consequence that a smaller number of microstructured components can be produced in a certain unit of time. Furthermore, such a measuring method can have the disadvantage that the wavefront aberrations of the projection objective can change during the exposure of the photosensitive layer and this is only ascertained upon the next measurement interruption.